President Saddam, it is argued, can be treated as a fair target because he is chief of the Iraqi military at a time of war. "International human rights law allows you to target military commanders," said Jim Ross, the senior legal analyst at Human Rights Watch in New York. "The Geneva convention gives a great deal of leeway when it comes to attacking combatants."

If the US agreed to a truce whereby President Saddam gave himself up in return for an end to military action, it would be illegal to kill him, Mr Ross said. Under the Geneva convention they must minimise civilian casualties when in pursuit of a foreign leader.

It is precisely the fear of a retaliatory attack on an American leader that prompted President Gerald Ford to issue a ban on assassinations of heads of state in 1976. Executive order No 12333 declares: "No person employed by or acting on behalf of the United States government shall engage in, or conspire to engage in, assassination."

Mr Ford's ruling has been signed by every president since, but many have sought to sidestep it. In 1986 President Ronald Reagan ordered air strikes on the presidential compound of the Libyan leader, Muammar Gadafy. Two years later the ruling was "reinterpreted" under George Bush senior in order to target the Panamanian leader, Manuel Noriega. The new understanding was that the prohibition did not apply if a leader was killed as an unintended consequence of an action undertaken or supported by the government.

George Bush junior then further broadened his authority after the terrorist attacks on September 11. In a secret directive he authorised the CIA to target suspected terrorists for killing, when capture was impractical and civilian casualties could be minimised.